The field of the invention is programmable controllers such as those disclosed in U.S. Pat. Nos. 4,165,534; 4,266,281; and 4,291,388.
Programmable controllers are employed in many industrial and commercial applications to control the operation of various types of machines. Programmable controllers are characterized by the repeated execution of a stored control program which contains instructions that direct the controller to examine the status of various sensing devices on the controlled machine and to operate various output devices on the controlled machine.
The size, or capacity, of a programmable controller should be compatible with the size of the machine or process being controlled. Size is typically measured by the number of I/O points the controller can support without degrading its response time to changing conditions on the controlled machine or process. In many applications this goal is met initially with a relatively small capacity programmable controller, but with the passage of time (and further automation) the capacity of the programmable controller is sometimes exceeded.
One solution to this problem is disclosed in U.S. Pat. No. 4,250,563 where the processor in a low capacity programmable controller may be easily replaced with a more powerful processor which will support additional I/O points. Another solution which is applicable to some installations is disclosed in co-pending U.S. Pat. No. 4,319,338 entitled "Industrial Communications Network With Mastership Determined By Need". The industrial communications network enables programmable controllers to be connected together through a high speed data link. Although this network has very effectively enabled programmable controllers to communicate with each other, and has thereby facilitated the addition of controllers as the control task grows, the hardware required for such communications networks is relatively expensive.
In some applications the sensing devices and operating devices on the machine or process being controlled are distributed over a wide area. In some installations it may be more desirable to employ a plurality of small programmable controllers distributed throughout the facility rather than a single large programmable controller. To implement such a distributed processing strategy, an economical means for connecting the programmable controllers into a single communications network is required.